Various types of injection devices have been described over the years. However, injection devices formed as pen-type injection devices have attracted special attention over recent years.
For example, EP 0 338 806 discloses pen-like syringe incorporating a dose metering device. The pen is provided with a cap rotatable, with respect to a pen body, to a position representing the dose of medicament (for example insulin) to be injected. The rotation loads a torsion spring, which is prevented from unwinding by co-operating teeth. When the dose is to be injected, a trigger slide is moved to the left causing the toothing to come out of engagement. This disengagement allows the spring to unwind. The unwinding of the spring causes a drive sleeve, drive gear and a drive plunger to rotate. The drive plunger is formed with a quick pitch screw thread so that its rotational movement is accompanied by an axial movement which causes medicament to be discharged from a cartridge and injected through a needle.
EP 0 338 806 is based on a torsion spring. However, by applying a linear spring the axial movement of an integrated dose metering device could be made independent of the pitch screw threads on the plunger and the integrated dose metering device would be capable of rotating more than one revolution following a helical pattern.
U.S. Pat. No. 6,673,035 discloses a medical injector and medicament loading system for use therewith. The medicament loading system includes cap for a medicament cartridge. The cap has a post for causing movement of the cartridge stopper toward the seal when the cap engages the medicament cartridge to thereby eliminate adhesion between the medicament chamber and the stopper. The medical injector according to the present invention includes the medicament loading system, i.e. a cartridge assembly, a needle free syringe assembly, and a power pack assembly.
WO 02/053214 relates to a dose setting and expelling device comprising a drive member and a dose setting mechanism which simultaneously sets a given dose and stores the energy necessary for subsequently driving the drive member in order to expel a dose of medicine from an injection device. According to WO 02/053214 the dose setting mechanism allows adjustment in both directions, such that a given set dose can be reduced or cancelled by reversing the input motion, typically by rotating a setting member in a backward direction. Thus, WO 02/053214 discloses a simple dose setting and reverse mechanism. However, it is a disadvantage of the arrangement suggested in WO 02/053214 that the axial movement of an integrated dose metering device is dependent on the pitch screw threads on the plunger.
It is an object of the present invention to provide an injection device where the axial movement of the dose metering device is independent of the pitch of the threads on the plunger.
It is a still further object of the present invention to provide an injection device with a dose metering device allowing larger read-outs compared to known systems.